Unprecedented Intramolecular [4 + 2]-Cycloaddition between a 1,3-Diene and a Diazo Ester

Diazo
compounds that are well-known to undergo [3 + 2]-cycloaddition
provide the first examples of the previously unknown [4 + 2]-cycloaddition
with dienes that occur thermally under mild conditions and in high
yields. Reactions are initiated from reactants prepared from propargyl
aryldiazo­acetates that undergo gold­(I)-catalyzed rearrangement
to activated 1,3-dienyl aryldiazo­acetates. These reactions proceed
to mixtures of both [4 + 2]-cycloaddition and the 1,3-dienyl aryldiazo­acetate
after long reaction times. At short reaction times, however, both E- and Z-1,3-dienyl aryldiazo­acetates
are formed and, after isolation, thermal reactions with the E-isomers form the products from [4 + 2]-cycloaddition with
ΔH‡298 = 15.6
kcal/mol and ΔS‡298 = −27.3 cal/(mol·°C). The Z-isomer
is inert to [4 + 2]-cycloaddition under these conditions. The Hammett
relationships from aryl-substituted diazo esters (ρ = +0.89)
and aryl-substituted dienes (ρ = −1.65) are consistent
with the dipolar nature of this transformation.